Method of producing ultra-hard abrasive particles

ABSTRACT

A method of producing a plurality of discrete ultra-hard abrasive particles includes the steps of providing a plurality of granules, each comprising at least one ultra-hard abrasive particle, a precursor for the abrasive particle and a solvent/catalyst for the abrasive particle or precursor of such a solvent/catalyst, placing the granules with a separating medium between adjacent granules in the reaction zone of a high pressure/high temperature apparatus, subjecting the contents of the reaction zone to elevated temperature and pressure conditions at which the ultra-hard abrasive particle is crystallographically stable, recovering thus treated material from the reaction zone and removing the separating medium in the treated material to produce a plurality of discrete abrasive particles.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a method of producing ultra-hardabrasive particles, particularly diamond particles.

[0002] Methods of producing diamond and cubic boron nitride abrasiveparticles synthetically are well known in the art. The methods can betailored to produce particles having particular characteristics. Forexample, the method may be tailored to produce friable diamond particleswhich are used in applications such as grinding. Alternatively, themethod may be tailored to produce a strong blocky diamond of goodquality. Such diamonds are typically used in saws and grindingapplications.

[0003] Diamonds are synthesised by subjecting a carbon source i.e. aprecursor of diamond, to elevated temperature and pressure conditions atwhich diamond is crystallographically stable, generally in the presenceof a diamond solvent catalyst. Similarly, cubic boron nitride particlesare synthesised by subjecting hexagonal boron nitride, i.e. theprecursor of cubic boron nitride, to elevated temperature and pressureconditions at which cubic boron nitride is crystallographically stablein the presence of a solvent/catalyst for cubic boron nitride.

[0004] EP 737510 describes a method of synthesising diamond particles bycoating fine diamond particles with at least one layer of a non-diamondcarbon material, a catalyst/solvent in the form of a metal powder and anorganic binder, compacting the coated particles in such a manner thatthey are at least partially in contact with each other, placing thecompacted arrangement in a suitable synthesising vessel and subjectingthe compacted arrangement to temperature and pressure conditions atwhich diamond is crystallographically stable.

[0005] The diamond which is synthesised is a relatively large diamond.This method of producing relatively large diamonds has severaldrawbacks. First, the fine particles are difficult to coat, particularlyusing the fluidisation method described in the patent. As a consequence,agglomeration of the seeds tends to occur. If larger particles are usedas the seed, remnants of the seed can appear visibly in the synthesiseddiamond. Further, the seeds in the compacted arrangement are exposed toa variable environment during the growth stage. This variability resultsin variability in the diamond synthesised.

SUMMARY OF THE INVENTION

[0006] According to the present invention, a method of producing aplurality of discrete ultra-hard abrasive particles includes the stepsof providing a plurality of granules, each comprising at least oneultra-hard abrasive particle, a precursor for the abrasive particle anda solvent/catalyst for the abrasive particle or precursor of such asolvent/catalyst, placing the granules with a separating medium betweenadjacent granules in the reaction zone of a high pressure/hightemperature apparatus, subjecting the contents of the reaction zone toelevated temperature and pressure conditions at which the ultra-hardabrasive particle is crystallographically stable, recovering the thustreated material from the reaction zone and removing the separatingmedium in the treated material to produce a plurality of discreteabrasive particles.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIGS. 1 to 4 illustrate diagrammatically the steps of anembodiment of the method of the invention.

DESCRIPTION OF THE EMBODIMENTS

[0008] The method of the invention involves, as a first step, producinga plurality of granules. Each granule will contain an ultra-hardabrasive particle and preferably only one such particle. The granulealso contains solvent/catalyst for the ultra-hard abrasive particle or aprecursor of such a solvent/catalyst and a precursor for the ultra-hardabrasive particle. The granules will be a coherent mass of the variouscomponents in any suitable shape or size and may be produced by methodssuch as granulation, pelletising or spray coating.

[0009] The granules may contain an organic or inorganic binder. Examplesof such binders are cellulose ethers, organic polymers and the like.Such binders will generally be removed prior to subjecting the granulesto the high temperature/high pressure growth conditions.

[0010] The ultra-hard abrasive particles will generally be diamond orcubic boron nitride particles. The method has particular application toproducing diamond particles. The particles in the granules willgenerally be fine, e.g. have a size of less than 100 microns. Asmentioned above, the preferred form of the invention is that eachgranule contains a single ultra-hard abrasive particle only.

[0011] The solvent/catalyst or precursor thereof and the precursor forthe ultra-hard abrasive particle may be provided in layer form or as amixture in each granule, the latter being preferred. These componentswill generally be in powder form in the granules.

[0012] Solvent/catalysts for diamond and cubic boron nitride are wellknown in the art. Particularly suitable examples for diamondsolvent/catalysts are transition metals such as cobalt, iron, nickel oralloys containing one or more of these metals. A precursor of thesolvent/catalyst may also be used. Examples of diamond solvent/catalystprecursors are oxides such as nickel oxide, cobalt oxide or iron oxideor compounds which reduce or pyrolise to an oxide such as carbonates andoxalates of metals such as iron, cobalt or nickel. When precursors areused, it is preferred that the granules are subjected to a heattreatment to reduce the precursors to the metal prior to subjecting thegranules to the high temperature/high pressure sintering. The heattreatment for the reduction will vary according to the nature of thegranules, its content and the nature of the precursor. The precursors ofthe solvent/catalyst reduce to the metal in a particularly fine particlesize such that a finely divided and homogeneous mixture of thecomponents of the layer around the ultra-hard abrasive particle isprovided.

[0013] The precursor for diamond will be a non-diamond carbon such asgraphite or amorphous carbon. The precursor for cubic boron nitride willbe hexagonal boron nitride.

[0014] In one form of the invention, the ultra-hard abrasive particlesare each coated with a layer of solvent/catalyst prior to forming thegranules. Preferably, the solvent/catalyst, when it is a metal, isapplied to the abrasive particle by electroless coating.

[0015] A separating medium is provided between adjacent granules whenthey are in the reaction zone of the high pressure/high temperatureapparatus. Preferably, the separating medium is applied as a coating tothe granules prior to placing the granules in the reaction zone. Thecoating may be applied by methods known in the art. For example, thecoating may be provided by spray coating in a fluidised medium. Theseparating medium must be of a material which does not interfere withthe high temperature/high pressure treatment of the contents of thegranules. Preferably the separating medium has the ability to transmitadequately the pressure which is generated in the reaction volume. It isalso preferable that the separating medium is readily removed from thegranule after treatment by dissolution in water or a dilute acid,optionally with the aid of sonification, or by mechanical means.Further, it is preferable that the separating medium is a good thermalconductor. Examples of suitable separating media are salt, graphite,hexagonal boron nitride, carbonates, oxides, phosphates and the like.The separating medium, in the case of diamond, can also be a metal suchas molybdenum. The separating medium may comprise more than one layer.

[0016] The separating medium may be provided between adjacent granulesin other ways. For example, the granules may be placed in a container,e.g. a capsule, and the separating medium, in powdered or liquid form,poured into the container and around the granules. If necessary, gooddispersion and mixing can be assisted by some mechanical agitation orvibration to ensure that there are no gaps and the granules are allembedded and uniformly dispersed in the separating medium.

[0017] The separating medium ensures that granules are separated andthat the ultra-hard abrasive particle in each granule is thus exposed toa uniform growth medium. Consequently, a more uniform and consistentdiamond growth occurs. The separating medium provides a reaction barrieraround each granule, and thereby also reduces the effect of temperaturegradients in the reaction zone which otherwise result in inhomogeneouscrystallisation throughout the reaction zone.

[0018] The granules may be selected on the basis of size. For example,granules of a preferred size may be selected by sieving from a bulk ofgranules, or by means of a cyclone or any other suitable sizeclassification method.

[0019] Generally, the granules and separating medium, as a coating orotherwise, will be placed in a reaction capsule and pressed or compactedinto the capsule to fill the space available.

[0020] The elevated temperature and pressure growth conditions to whichthe granules are subjected are well known in the art. Typical pressuresare in the range of 3 to 8 GPa and typically temperatures are in therange of 1000 to 2100° C.

[0021] The treated material is removed from the reaction zone of thehigh temperature/high pressure apparatus. The material which isrecovered is sintered and coherent and consists of a plurality oftreated granules separated by the separating medium. The discreteindividual ultra-hard abrasive particles can be released by firstremoving the separating medium and then removing residualsolvent/catalyst and precursor. These recovery steps are known in theart.

[0022] An embodiment of the invention will now be described withreference to the accompanying drawing. Referring first to FIG. 1, adiamond seed 10, typically having a size of less than 100 microns, iscoated with a layer 12, which completely encloses the seed 10, of ametal solvent/catalyst. An example of such a metal is nickel. Thiscoating preferably takes place by electroless coating.

[0023] The coated diamond of FIG. 1 is then provided with a coating ofsolvent/catalyst or precursor thereof and a separating medium. One suchmethod of coating is illustrated by FIG. 2. Referring to this Figure, acontainer suitable for fluidisation is provided. A mass 20 of the coatedseeds of FIG. 1 is placed in the container and air or other gasintroduced through the base 22 of the container 18 as shown by arrows 24so as to fluidise the mass 20. First, a stable suspension ofsolvent/catalyst or precursor and graphite is made, and a layer of thesuspension sprayed on to the fluidised particles through spraying unit26. Thereafter, through the same spraying unit 26, a separating mediumis applied to the fluidised particles. A coated particle or granule isproduced.

[0024] An example of the coated particle or granule which is produced isillustrated by FIG. 3. Referring to this Figure, the coated particle orgranule comprises diamond seed 10, a solvent/metal coating 12, a layer14 of solvent/catalyst or precursor and graphite and an outer layer 16of a separating medium. If a precursor of the solvent/catalyst is used,then the granules may be treated to reduce the precursor to thesolvent/catalyst prior to the next step. The solvent/catalyst 12 may bedifferent to the solvent/catalyst of layer 14.

[0025] Referring to FIG. 4, the granules 28 are placed in a reactioncapsule 30 of a high pressure/high temperature apparatus together with athird phase or transmitting medium 32. The contents of the capsule arecompacted to ensure that there are no spaces or gaps. The loaded capsuleis then placed in the reaction zone of the high temperature/highpressure apparatus and subjected to diamond synthesis conditions. Underthese conditions, graphite dissolves in the solvent/catalyst andmigrates to the diamond seed. Growth on the diamond seed takes place.The diamond seed in each granule, encapsulated and isolated as it is, isexposed to a uniform surrounding composition of solvent/catalyst andgraphite and thus uniform growth on the diamond takes place. The diamondwhich is recovered from each granule is large and of good crystal growthand shape.

1. A method of producing a plurality of discrete ultra-hard abrasiveparticles includes the steps of providing a plurality of granules, eachcomprising at least one ultra-hard abrasive particle, a precursor forthe abrasive particle and a solvent/catalyst for the abrasive particleor precursor of such a solvent/catalyst, placing the granules with aseparating medium between adjacent granules in the reaction zone of ahigh pressure/high temperature apparatus, subjecting the contents of thereaction zone to elevated temperature and pressure conditions at whichthe ultra-hard abrasive particle is crystallographically stable,recovering thus treated material from the reaction zone and removing theseparating medium in the treated material to produce a plurality ofdiscrete abrasive particles.
 2. A method according to claim 1 whereineach granule contains a single ultra-hard abrasive particle only.
 3. Amethod according to claim 1 or claim 2 wherein the solvent/catalyst orprecursor thereof and the precursor of the ultra-hard abrasive particleare provided in layer form or as a mixture in the granules.
 4. A methodaccording to any one of the preceding claims wherein thesolvent/catalyst or precursor thereof and the precursor of theultra-hard abrasive particle are all provided in powder form in thegranules.
 5. A method according to any one of the preceding claimswherein the ultra-hard abrasive particle is selected from diamond andcubic boron nitride.
 6. A method according to any one of the precedingclaims wherein the ultra-hard abrasive particle is diamond and aprecursor of a diamond solvent/catalyst is present in the granules.
 7. Amethod according to claim 6 wherein the precursor is an oxide of a metalsolvent/catalyst or a compound which reduces or pyrolises to an oxide ofa metal solvent/catalyst.
 8. A method according to claim 7 wherein theprecursor is selected from iron oxide, cobalt oxide and nickel oxide anda mixture thereof.
 9. A method according to claim 7 or claim 8 whereinthe granules are heat treated to reduce the precursor to the metal priorto subjecting the granules to the high temperature/high pressuresintering.
 10. A method according to any one of the preceding claimswherein the ultra-hard abrasive particle in at least some of thegranules is provided with a layer of solvent/catalyst prior to formingthe granules.
 11. A method according to claim 10 wherein thesolvent/catalyst is a metal solvent/catalyst and is applied to theabrasive particle by electroless coating.
 12. A method according to anyone of the preceding claims wherein the separating medium is applied asa coating to the granules prior to placing the granules in the reactionzone.
 13. A method according to claim 12 wherein the coating is appliedby spray coating the ultra-hard abrasive particles in a fluidisedmedium.
 14. A method according to any one of the preceding claimswherein the separating medium is selected from salt, graphite, hexagonalboron nitride, a carbonate, an oxide, a phosphate and the like.
 15. Amethod according to any one of the preceding claims wherein theultra-hard abrasive particle is diamond and the separating medium ismolybdenum.
 16. A method according to any one of the preceding claimswherein the elevated temperature and pressure sintering conditions are apressure of 3 to 8 GPa and a temperature of 1000 to 2100° C.
 17. Amethod according to claim 1 substantially as herein described withreference to FIGS. 1 to 4 of the accompanying drawing.